Method and device for coding an audio signal by “forward” and “backward” LPC analysis

ABSTRACT

A method and device for encoding a digital audio-signal divided into a succession of blocks according to a LPC &#34;forward&#34; and &#34;backward&#34; analysis respectively under a choice criterion. For coding each current block, the choice criterion is established by defining the degree of stationarity of the digital audio-signal according to a stationarity parameter belonging to a maximum and a minimum stationarity range value. An analysis choice value is established from a decision function and this stationarity parameter and thus applied to the digital audio-signal to have this audio digital signal encoded by &#34;backward&#34; LPC filtering for stationary zones. The &#34;forward&#34; and &#34;backward&#34; filtering mode are thus performed in relation to the degree of stationarity of the audio digital signal, the amount of switching from one to the other filtering modes being thus limited.

FIELD OF THE INVENTION

The invention involves a procedure and a device for coding anaudio-frequency signal, such as a speech signal, by means of “forward”and “backward” LPC analysis.

At present, the aim of coding techniques for audio-frequency signals,particularly speech signals, is to allow for the transmission of thesesignals in digital form, within the conditions of reduction of thetransmission output, in order, particularly, to ensure a managementadapted to the networks for transmitting these signals, taking intoconsideration the considerable growth in transactions between users.

BACKGROUND OF THE INVENTION

Of the coding techniques used, that designated by LPC analysis, LinearPredictive Coding in English, consists of carrying out a linearprediction of the audio-frequency signal to be encoded, the coding beingcarried out temporarily by means of a linear filtering predictionapplied to the successive blocks of this signal.

Of the aforementioned techniques, that known as CELP coding, CodeExcited Linear Prediction, is the most widespread and provides some ofthe best performance. Other techniques, such as the technique designatedby MP-LPC, Multi Pulse Linear Predictive Coding, or the VSELP technique,Vector Sum Excited Linear Prediction in English, are relatively similarto CELP coding.

The aforementioned coding techniques are known as “analysis bysynthesis”. They have enabled in particular, for audio-frequency signalsbelonging to the telephonic frequency bandwidth, the transmission outputof these signals to be reduced from 64 kb/s (MIC coding) to 16 kb/s withthe help of the CELP coding technique and even to 8 kb/s where theseencoders use the most recent developments of this coding technique,without any perceptible reduction in the quality of the voicereconstituted after transmission and decoding.

A particularly important area of application for these coding techniquesis, in particular, that of mobile telephony. Within this area ofapplication, the necessary limitation of the frequency bandwidth grantedto each mobile-telephony operator and the extremely rapid increase inthe number of subscribers makes necessary the corresponding reduction ofthe coding output, while user demands in terms of speech qualitycontinue to grow. Other areas of application of these coding techniquesconcern, for example, the storage of digital data which represent thesesignals on memory supports, high-quality telephony for video or audioconference applications, multimedia or digital transmissions viasatellite.

The linear prediction filters used in the aforementioned techniques areobtained with the help of an analysis module called “LPC analysis”operating on successive digital signal blocks. These filters arecapable, according to the order of analysis, that is, according to thenumber of filter coefficients, of modeling more or less reliably thecontours of the spectrum of frequencies of the signal to be coded. Inthe case of a speech signal, these contours are called formants.

However, for good quality coding, required by most current applications,the filter thus defined is not sufficient for perfectly modeling thesignal. It is therefore essential to code the residue of the linearprediction. One such operating mode relating to linear predictionresidue is particularly used by the coding technique, LD-CELP, Low DelayCELP in English, previously mentioned in the description. In this case,the residual signal is modeled by a waveform taken from a stochasticcodepage and multiplied by a gain value. The MP-LPC coding technique,for example, models this residue with the help of variable positionpulses modified by respective gain values, whereas the VSELP codingtechnique carries out this modeling by means of a linear combination ofpulse vectors taken from appropriate lists.

An explanatory recap of the operating method of LPC analysis andespecially “backward” LPC analysis and “forward” LPC analysis will begiven below.

The general envelope of the frequency spectrum is modeled by means of ashort-term synthesis filter, constituting the LPC filter, thecoefficients of which are modeled by means of a linear prediction of thespeech signal to be coded. This LPC filter, an autoregressive filter,has a transfer function of the form, equation (1):${A(z)} = {1 - {\sum\limits_{i = 1}^{p}{a_{i}z^{- i}}}}$

where p designates the name of coefficients, ai of the filter and theorder of the linear prediction applied, z designating the transformedvariable z of the space of the frequencies.

One method of evaluating the coefficients a_(i) consists of applying acriterion of minimization of the energy of the error prediction signalof the speech signal over the analysis length of this latter.

The analysis length for a digital speech signal formed of successivesamples is, in practical terms, a number N of these samples,constituting a coding frame. The energy of the error prediction signalthus confirms equation (2):${Ep} = {\sum\limits_{n = 1}^{N}\left( {{s(n)} - {\sum\limits_{i = 1}^{p}{a_{i} \cdot {s\left( {n - 1} \right)}}}} \right)^{2}}$

where s(n) designates the sample of row n in the frame of N samples.

In a block-by-block coding process, the coding frame can beadvantageously divided into several subframes or adjacent LPC blocks.The analysis length N then exceeds the length of each block in order tomake it possible to take into account a certain number of past or, ifapplicable, future samples, by means of and at the cost of delaying theappropriate coding.

The analysis is called “forward” LPC when the LPC analysis process iscarried out on the block of the current frame of the speech signal to becoded, with the coding taking place at encoder level “in real time”,that is, during the block of the current frame, with the only processingdelay introduced by the calculation of the filter coefficients. Thisanalysis involves transmitting the calculated values of the filtercoefficients to the decoder.

“Backward” LPC analysis, used in the LD-CELP encoder at 16 kb/s is theobject of the standard UIT-T G728. This analysis technique consists ofcarrying out the LPC analysis not on the block of the current frame ofthe speech signal to be coded, but on the synthesis signal. It isunderstood that this LPC analysis is actually performed on the synthesissignal of the block preceding the current block, as this signal isavailable simultaneously at encoder and decoder level. This simultaneousoperation in the encoder and decoder thus makes it possible to avoidtransmitting from the encoder to the decoder the value obtained in theencoder of the LPC filter coefficients. For this reason, “backward” LPCanalysis makes it possible to free up transmission output and the outputthus freed can be used, for example to enrich the excitation codepagesin the case of CELP coding. “Backward” LPC analysis furthermore allowsan increase in the order of analysis; the number of LPC filtercoefficients may be as much as 50 in the case of an LD-CELP encoder,compared to 10 coefficients for most encoders using “forward” LPCanalysis.

Thus, correct operation of “backward” LPC analysis requires thefollowing conditions:

good quality synthesis signal, very close to the speech signal to becoded, which involves a sufficiently high coding output, higher than 13kb/s, taking into account the quality of current CELP encoders;

reduced frame and block length due to the delay of one block between theanalyzed signal and the signal to be coded. The length of the frame andblock should therefore be low in comparison to the mean stationary timeof the speech signal to be coded;

reliability of the transmission and conservation of the integrity of thedata transmitted between the encoder and the decoder, by introducing fewtransmission errors. As soon as the synthesis signals differsignificantly from the speech signal to be coded, the encoder anddecoder cease to calculate the same filter and large divergences mayoccur, without being able to return to a noticeable similarity of thefilters calculated in the encoder or decoder.

Due to the respective advantages and disadvantages of the aforementioned“backward” and “forward” types of LPC analysis, one technique consistingof selectively associating “backward” and “forward” LPC analysis wasproposed in the article titled “Dual Rate Low Delay CELP Coding (8kbits/s/16 kbits/s) using a Mixed Backward/Forward Adaptive LPCPrediction”, published by S. PROUST, C. LAMBLIN and D. MASSALOUX, Proc.IEEE Workshop Speech Co. Telecomm., September. 1995, pp 37-38.

The conditions mentioned above, regarding the correct functioning of“backward” LPC analysis, show that this type of analysis alone presentsthe limitations mentioned when operating at transmission outputsappreciably below 16 kb/s. Besides the reduction in the quality of thesynthesis signal, which reduces the performance of the LPC filter, it isvery often necessary, in order to reduce the transmission output, tooperate with a greater LPC frame length, of the order of 10 to 30 ms. Itcan therefore be seen that, under these conditions, the degradationoccurs especially during transitions of the frequency spectrum and, moregenerally, in the not so stationary areas, since for generally verystationary signals, such as music signals, “backward” LPC analysis holdsa considerable advantage over “forward” LPC analysis.

The association of the two aforementioned types of LPC analysis aims toreduce these disadvantages and increase the advantages inherent in eachone:

“forward” LPC analysis for the coding of the transitions and thenon-stationary areas;

“backward” LPC analysis, to a greater extent, for the coding of thestationary areas.

Furthermore, the introduction of LPC frames coded by “forward” LPCanalysis into LPC frames coded by “backward” analysis allows the encoderand decoder to re-converge towards the same synthesis signal in the caseof a transmission error and therefore offers far greater errorprotection than coding by “backward” LPC analysis alone.

In general, the above-mentioned mixed “forward”-“backward” LPC analysisconsists of carrying out two LPC analyses, a “forward” LPC analysis ofthe speech signal or audio frequency to be coded and a “backward” LPCanalysis of the synthesis signal.

Two filters are calculated for each LPC block, these filters beingdesignated by “forward” LPC filter and “backward” LPC filter,respectively. A procedure of choosing the filter applied to the LPCblock, depending on whether the signal is stationary, is thereforeapplied. This procedure requires two different criteria:

a first criterion based on the prediction gains of the filters;

a second criterion based on a distance parameter between the “forward”LPC filters calculated successively.

For each of these two criteria, the threshold values are established.

First Criterion:

The choice of “backward” LPC filter is made if the distance between theprediction gain of the “backward” and “forward” LPC filters is greaterthan a first threshold value.

Second Criterion:

For a current analysis in “backward” LPC analysis mode, prohibition ofswitching from “backward” LPC analysis mode to “forward” LPC analysismode if the distance calculated on the vectors of the parametersrepresenting two consecutive “forward” LPC filters is lower than asecond threshold value, a distance which is too small characterizing amore or less stationary area, for which reason it is appropriate toavoid changing the LPC analysis mode. The calculated distance is aEuclidean distance between the spectral lines of the speech oraudio-frequency signal to be coded.

A more detailed description of the aforementioned mixed LPC analysismethod can be found in the article published by S. PROUST, C. LAMBLINand D. MASSALOUX, mentioned above.

In-depth studies on the above-mentioned mixed analysis operating methodhave shown the following important disadvantages:

for certain signals, the prediction gain values of the “forward” and“backward” LPC filters may oscillate above and below the first thresholdvalue. This phenomenon leads to sudden and frequent changes from“backward” LPC filter to “forward” LPC filter or vice versa. Thediscontinuity of filtering thus introduced constitutes a source ofconsiderable degradation of the synthesis signal and is not, most of thetime, linked to the real spectral modifications of the speech oraudio-frequency signal to be coded;

the optimal value of the first threshold which should be establishedvaries considerably according to whether the signal to be coded isstationary, more so when the coding output is low. For a coding delaycorresponding to an LPC frame of 10 to 30 ms, or when the transmissionoutput falls, there is a clear divergence between the coding mode ofmusical signals and speech signals; “forward” LPC analysis is mainlyused.

Since music signals are quite stationary, “backward” LPC analysis isused even for long LPC frames. In the case of speech signals, however,the highly stationary areas have a very short duration and their passagein “backward” LPC analysis mode is therefore brief, thus leading tounwanted filter transitions which reduce the quality of the coding. Theencoder can thus no longer correct the phenomena generated by thediscontinuity introduced by the switching of the filters.

The LPC filter which gives the best subjective quality and whichtherefore best models the spectrum of the signal to be coded is notalways that which has the best prediction gain. Certain switchings fromone mode of LPC analysis to another, linked to an instantaneousdecision, are therefore useless.

SUMMARY OF THE INVENTION

The object of the present invention is to resolve the aforementioneddisadvantages by employing a procedure and device for coding a digitalaudio-frequency signal by means of specific “forward” and “backward” LPCanalysis.

Another object of the present invention is also to employ a process fordynamically adapting the function of choice between “forward” LPCanalysis and “backward” LPC analysis according to how stationary thesignal to be coded is.

A further object of the present invention is also to employ a processfor dynamically adapting the aforementioned choice function on the basisof discrimination between highly stationary signals, such as music orbackground noise, and other signals, such as speech, in order to allowthe most appropriate code processing by “backward” LPC analysis and“forward” LPC analysis, respectively.

A further object of the present invention is, once the aforementionedmost appropriate choice of coding has been made, for a signal to becoded of a given type or with given characteristics, to prevent anysudden switching to the LPC analysis mode not chosen and, therefore, toprevent the appearance of transitions from “forward” LPC filters to“backward” LPC filters and vice versa, which tend to reduce the qualityof the reproduced synthesis signal.

A further object of the present invention is to employ a dynamicadaptation process of the aforementioned choice function by which thechange in the LPC analysis mode corresponds reliably to a change in thestationarity of the signal to be coded, thus having a far lower chanceof being linked to a simple crossover effect of the first and secondthreshold values.

The method and device for coding a digital audio-frequency signal, whichare the object of the present invention, employ a double analysis basedon the criterion of choice between “forward” and “backward” LPCanalysis, respectively, to create a transmitted coded signal consistingof LPC filtering parameters accompanied by analysis decision informationand a non-transmitted coding residue signal. The digital audio-frequencysignal is subdivided into frames, succession of blocks of a determinednumber of samples, and the coding of this digital audio-frequency signalis carried out on this signal using a “forward” LPC filter for thenon-stationary areas and a synthesis signal, respectively. Thissynthesis signal is obtained from the coding residue signal, using“backward” LPC filtering for the stationary areas.

They are notable insofar as they consist of and allow for, respectively:

determining the degree of stationarity of the digital audio-frequencysignal according to a stationarity parameter whose value is between amaximum stationarity value and a minimum stationarity value;

establishing, based on the stationarity parameter, an analysis choicevalue, based on a decision function;

applying the analysis choice value to the LPC filtering in order to codethe digital audio-frequency signal by means of “forward” LPC filteringon the non-stationary areas of the digital audio-frequency and by meansof “backward” LPC filtering on the stationary areas of the synthesissignal.

This operating method makes it possible to prioritize remaining ineither the “forward” or “backward” LPC filtering mode, according to thedegree of stationarity of the digital audio-frequency signal and tolimit the number of switchings from one mode of filtering to another andvice versa.

The method and the device which are the object of the present inventionhave an application not only in the area of mobile telephony, but alsoin the sector of creation and reproduction of phonograms, satellitetransmission and high-quality telephony for multimedia video or audioconference applications.

BRIEF DESCRIPTION OF THE DRAWINGS

Understanding will be facilitated by reading the description andexamining the design below, where:

FIG. 1 shows, in the form of a general flow chart, an explanatorydiagram of the stages which allow the performance of the coding which isthe object of the present invention;

FIG. 2a shows a general flow chart of the stages of calculating thestationarity parameter for each current LPC block;

FIG. 2b shows a particularly advantageous method of carrying out theessential stages of the calculation of the stationarity parameter,according to FIG. 2a;

FIG. 2c shows a detail of the execution of FIG. 2b and, moreparticularly, a detail of the process of tuning the value of theintermediate stationarity parameter in order to obtain the stationarityparameter;

FIGS. 2d and 2 e show, respectively, a first and second example of theapplication of a tuning function, allowing for the calculation of atuning value for the intermediate stationarity function according to thecomparative values of the “forward” and “backward” LPC filter gain;

FIG. 2f shows as an explanatory example a flow chart of the stagesmaking it possible to employ the decision function and the “forward” or“backward” LPC analysis choice value;

FIG. 3 shows, in the form of functional blocks, the general diagram ofan encoder which makes it possible to code an audio-frequency signalaccording to the object of the present invention;

FIG. 4 shows, in the form of functional blocks, the general diagram of adecoder which makes it possible to decode an audio-frequency signalwhich has been coded by using an encoder as shown in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A more detailed description of the method for coding a digitalaudio-frequency signal, employing a double analysis based on thecriterion of choice between “forward” and “backward” LPC analysis,respectively, of a transmitted coded signal, which is the object of thepresent invention, is now given in connection with FIG. 1.

In general terms, it is shown that the transmitted coded signal, writtenas s_c_(n)(t), consists in part of the LPC filtering parametersaccompanied by LPC analysis decision information. Furthermore, anon-transmitted coding residue signal, res_(n)(t), is available forperforming the coding procedure.

The digital audio-frequency signal is subdivided into LPC frames, asuccession of LPC blocks, each block, for the sake of convenience of thedescription, being written as Bn and having a determined number ofsamples, N.

One aspect of the coding procedure which is the object of the presentinvention consists of carrying out the aforementioned coding of thedigital audio-frequency signal as described above using “forward” LPCfiltering for the non-stationary areas and for a synthesis signalobtained from the coding residue signal using “backward” LPC filteringfor the stationary areas.

A particularly notable aspect of the method which is the object of thepresent invention consists of, in order to establish the “forward” or“backward” LPC filter choice criteria for each current block of thesuccession of current blocks forming the current frame, as shown in FIG.1, each current block, written B_(n), being available in an initialstage 10, to determine in stage 11 the degree of stationarity of thedigital audio-frequency signal, according to a stationarity parameter,written STAT(n). This stationarity parameter presents a digital valuebetween a maximum stationarity value, written STAT_(M), and a minimumstationarity value, written STAT_(m).

By way of convention and without prejudice to the degree of generalityof the coding procedure which is the object of the present invention,the stationarity parameter presents the maximum value STAT_(M) for anextremely stationary signal, whereas this stationarity parameterpresents the minimum value STAT_(m) for a highly non-stationary signal.

After the aforementioned stage 11, the coding method which is the objectof the present invention consist of establishing, in stage 12, using thestationarity parameter STAT(n), an LPC analysis choice value. Thisanalysis choice value corresponds, logically, to either the “forward”LPC analysis choice or the “backward” LPC analysis choice. The value ofthe choice of analysis is written d_(n)(n) and is obtained from aspecific decision function, written D_(n).

The aforementioned stage 12 is then followed by a test stage 13 whichallows the application of the analysis choice value d_(n)(n),represented by C, to the LPC filtering in order to carry out the codingof the digital audio-frequency signal by means of “forward” LPCfiltering for the non-stationary areas of the digital audio-frequencysignal and by means of “backward” LPC filtering for the stationary areasof the synthesis signal.

The execution of the decision function D_(n) and the aforementionedanalysis choice values d_(n)(n) form a particularly advantageous aspectof the coding procedure which is the object of the present invention, asthey make it possible to prioritize remaining in one of the LPCfiltering modes, either “forward” or “backward”, according to the degreeof stationarity of the audio-frequency signal and to limit the number ofswitchings from one to other of the filtering modes, and vice versa.

In general terms, it is mentioned that the decision function executed instage 12 and indicated as D_(n) is an adaptive function, updated foreach current block B_(n) from the stationarity parameter.

Updating the adaptive function makes it possible to prioritize remainingin one of the LPC filtering modes, either “forward” or “backward”,according to the degree of stationarity of the digital audio-frequencysignal and to hence limit the number of switchings from one to other ofthe filtering modes, and vice versa.

More specifically, the analysis choice value d_(n)(n) establishedaccording to the aforementioned decision function D_(n) corresponds to apriority value of the LPC filtering mode, either “forward” or“backward”, and to another priority value representing in fact a valueof absence of priority for returning to the “backward” or “forward” LPCfiltering mode.

As a priority value for the LPC filtering mode, it is mentioned that theanalysis choice value d_(n)(n) can, for example, correspond to a logicalvalue, the true value of this logical value, value 1, for example,corresponding to a choice of “backward” LPC filtering, whereas thecomplementary value of this true value, the value zero, corresponds to achoice of “forward” LPC filtering. It can thus be seen that the testfunction in stage 13 can be summarized as a test value of the logicalvalue of the aforementioned analysis choice value, to ensure in stage 14“backward” LPC filtering for the stationary areas of the signal to becoded or “forward” LPC filtering in stage 15 for the non-stationaryareas, the aforementioned stages 14 and 15 being thus followed by stages14 a and 15 a back to the next block, written as B_(n+1) for n=n+1.

Although the analysis choice value d_(n)(n) is represented by a logicalvalue, it is understood that this logical value may be associated with avalue of priority and probability of the mode of filtering specificallyestablished by the decision function D_(n). It can particularly be seenthat this probability value may correspond, for each current blockB_(n), to the true logical value for a range of probability valuesbetween zero and 1 for “backward” LPC filtering while the complementaryvalue, the logical value zero, for example, may correspond to thecomplement of the aforementioned range of probability values betweenzero and 1 for the first aforementioned range. This probability dependson a number of successive filtering decisions within the same filteringmode.

The operating mode of the decision function D_(n) makes it possible infact to associate with the logical variable d_(n)(n) the filtering modepriority and is adaptive over time for each current block B_(n).

In general terms, it is mentioned that the aim of adapting the decisionfunction D_(n) is to progressively prioritize the “backward” LPCfiltering mode or, in contrast, the “forward” LPC filtering mode,whichever works better, taking into account the overall stationarity ofthe signal to be coded, in order to avoid as far as possible anyunnecessary switching from one mode of filtering to another.

More specifically:

the more stationary the signal to be coded, the more the decisionfunction D_(n) prioritizes “backward” LPC analysis, limiting as far aspossible switching to “forward” LPC filtering mode;

in contrast, the less stationary the signal to be coded, the more thedecision function D_(n) prioritizes “forward” LPC analysis, limiting asfar as possible any switching to “backward” LPC filtering mode.

A more detailed description of the execution of the specific decisionfunction which makes it possible to adapt this decision function,according to the value of the stationarity parameter STAT(n), is givenlater in the description.

A method of preferential calculation of the stationarity parameterSTAT(n) relating to each current LPC block B_(n) is now given anddescribed in connection with FIG. 2a.

According to the aforementioned figure, stage 11, consisting ofdetermining the degree of stationarity of each current block B_(n) ofthe digital audio-frequency signal consists, starting with an arbitraryinitial value of the stationarity parameter, as shown in stage 110FIG.2a, this arbitrary value being written STAT(O), of calculating in stage111 for this current block B_(n), an intermediate stationarity parametervalue, written STAT*(n), as a function of a determined number ofsuccessive analysis choice values, these LPC analysis choice values,written d_(n−1)(n−1), . . . , to d_(n−p)(n−p), being obtained fordifferent successive blocks prior to the current block B_(n) of thesuccession of LPC blocks and a value of the stationarity parameter ofthe block preceding the current block, this stationarity value beingwritten STAT(n−1). In stage 111 shown in FIG. 2a, the function of thedetermined number of previous analysis choice values is given inrelation to these previous values, written d_(n−1)(n−1) to d_(n−p)(n−p).The initial arbitrary value for the stationarity parameter STAT(O) can,for example, be the same as the mean value between the maximum value andthe minimum value of the stationarity parameter mentioned above in thedescription, STAT_(M) and STAT_(m).

The aforementioned stage 111 is then followed by stage 112 whichconsists of tuning the intermediate stationarity parameter valueaccording to the value of the prediction gains of the “forward” and“backward” LPC filters or analysis mode of the frame preceding thecurrent frame. In stage 112 of FIG. 2a, the aforementioned function iswritten g(STAT*(n), Gpf, Gpb) where Gpf is the prediction gain of the“forward” LPC filter and Gpb is the prediction gain of the “backward”LPC filter for the frame preceding the current frame. In stage 112, thatis, the stage which consists of tuning the intermediate stationarityparameter value, the stationarity parameter value STAT(n) of the currentLPC block B_(n) is given the value, equation (3):

STAT(n)=g(STAT*(n), Gpf, Gpb)

corresponding to the tuned intermediate stationarity parameter value.

A more detailed description of the calculation stage 111 of theintermediate stationarity parameter STAT*(n) and of stage 112 whichconsists of tuning this parameter value is now given in connection withFIG. 2b.

According to the aforementioned figure, stage 111, starting with aninitialization stage 1110 in which the value of the stationarityparameter STAT(n−1) and the analysis choice value d_(n−1)(n−1) relatingto the LPC block B_(n−1) prior to the current block B_(n) is available,consists to carry out in stage 1111 a stage which consists ofdiscriminating the “forward” or “backward” LPC analysis mode of theblock B_(n−1) preceding the current block B_(n). This discriminationstage 1111, as shown in FIG. 2b, may consist of a test stage for theanalysis choice value d_(n−1)(n−1) in relation to the symbolic value“fwd” or the logical value zero, corresponding to the complementaryvalue of the true logical value.

On a negative response to the aforementioned test 1111, that is, for anyblock B_(n−1) preceding the current block B_(n), analyzed in “backward”LPC analysis mode, the stage which calculates the intermediatestationarity parameter value consists, in stage 1113, of determining thenumber of previous frames consecutively analyzed in “backward” LPCanalysis mode, written N_BWD; then, in stage 1114, it consists ofcomparing the superiority of the number of previous frames to an initialarbitrary value, written Na, representing a number of successive framesanalyzed in “backward” LPC mode.

On a positive response to the superiority comparison of test 1114, thecalculation stage consists of attributing in stage 1114 b, to theintermediate stationarity parameter value STAT*(n), the value of thestationarity parameter of the block preceding the current block,STAT(n−1), increased by a determined value which depends on the firstarbitrary value representing a number of successive analyzed frames,that is, the number of previous frames N_BWD, analyzed consecutively in“backward” LPC analysis mode. In stage 1114 b, the determined valuewhich depends on the first arbitrary value is written f_(n)(N_BWD).During the aforementioned stage, it can be seen that the intermediatestationarity parameter value STAT*(n) for the current LPC bloc B_(n) isthus increased in relation to the value corresponding to the samestationarity parameter for the preceding block B_(n−1).

On a negative response to the superiority comparison in the comparisontest 1114, the value of the stationarity parameter STAT(n−1) of theblock preceding the current block B_(n), is attributed, in stage 1114 a,to the intermediate stationarity parameter value STAT*(n).

However for every preceding bloc B_(n−1) analyzed in “forward” LPCanalysis mode, that is, on a positive response to test 1111, the stagefor calculating the intermediate stationarity parameter, 111, as shownin FIG. 2b, consists of determining in stage 1112, on the testcriterion, the occurrence of a transition from “backward” LPC analysismode to “forward” LPC analysis mode between the block before the blockbefore the current bloc B_(n−1), of row n−2, that is, the existence ofan LPC analysis choice value d_(n−2)(n−2)=symbolic value “bwd”, whoselogical value is zero, as mentioned above. The positive response to test1112 indicates the existence of such a transition from the “backward”analysis mode by the LPC block B_(n−1) preceding the block preceding thecurrent block B_(n−1), whereas a negative response to the aforementionedtest 1112 indicates the absence of such a transition.

On a positive response to the aforementioned occurrence test 1112, thecalculation stage 111 then consists of comparing using an inferioritycomparison criterion, the number of previous aforementioned N_BWD frameswith a second arbitrary value N_(b) which represents a number of framessuccessively analyzed in “backward” LPC mode preceding the bloc B_(n−1)preceding the current bloc.

On a positive response to the comparison performed in test 1118, thistest is followed by stage 1118 a, which consists of attributing to theintermediate stationarity parameter value STAT*(n) the stationarityparameter value of the block preceding the current block, STAT(n−1),reduced by a determined value which depends on the second arbitraryvalue N_(b); this determined value is written f₂(N_BWD). It can be seenthat in the attribution stage 1118 a, the intermediate stationarityparameter value is thus reduced as a result.

However on a negative response to the inferiority comparison carried outin test 1118, stage 111 consists then in allocating, in a stage 1118 b,to the value of the intermediate stationary parameter STAT*(n) the valueof the stationarity parameter of the block preceding the current block,i.e. STAT(n−1).

In FIG. 2b it will be noticed that the allocation stages 1118 a and 118b are then followed by a stage of replacing with zero the number ofsuccessive blocks processed in the “backward” LPC analysis mode, thisstage of making to zero carrying the reference 1118 c and enabling theupdating the whole of the calculation process of the value of theintermediate stationarity parameter.

On a negative response to the comparison test 1112, no “forward” LPCtransition analysis occurring, the value of the stationarity parameterSTAT(n−1) of the preceding block B_(n−1) is attributed to the value ofthe intermediate stationarity parameter STAT*(n) in a stage 1119.

At the end of stage 111, the value of the intermediate stationarityparameter STAT*(n) is set for the current block B_(n).

As far as the stage 112 consisting in tuning the value of theaforementioned intermediate stationarity parameter is concerned, it isnoted, by reference to FIG. 2b, that it consists to advantage, of astage 1120, in distinguishing the prediction gains of the “backward” LPCfiltering and the “forward” LC filtering, these gain values being notedGpb and Gpf respectively. It is understood that the aforementioneddiscrimination consists simply in memorizing and reading the gain valuescalculated for the respectively aforementioned “forward” and “backward”filtering. As well as the aforementioned gain values, the stage 1120 mayconsist of calculating the comparative value of the prediction gains,noted DGfb, as the difference or the ratio between the aforementioned“forward” and “backward” prediction gains.

As has been shown furthermore in FIG. 2b, the stage 112 of FIG. 2aincludes behind the aforementioned stage 1120 a stage 1121 consisting ofmodifying the value of the intermediate stationarity parameter STAT*(n)with a refining value ΔS, this refining value according to aparticularly noteworthy characteristic of the method which is the objectof the present invention being a function of the comparative value ofthe “forward” and “backward” LPC filtering prediction gains.

As a general rule, it is indicated that the function representative ofthe refining value ΔS is noted:

ΔS=f_(r)(Gpf, Gpb)

where Gpf and Gpb designate as previously the “forward” and “backward”LPC filtering prediction gains respectively.

As a general rule, it is indicated that the function f_(r)(GPf, Gpb)enabling the setting up of the refining value ΔS is a functionrespectively increasing and decreasing with this comparative value,according to the direction in which this comparative value isconsidered. When the comparative value designates the value of the“backward” LPC filtering gain comparative to the “forward” LPC filteringgain, this choice may be arbitrarily retained without any damage in thegeneral nature of the method, the object of the invention, to theaforementioned comparative value DGfb, the function fr is thenincreasing. It is decreasing in the opposite case.

In other terms, the modification, by increasing or decreasing, the valueof the intermediate stationarity parameter of the refining value ΔS isproportional to this comparative value of the gains. As a general rule,this modification is written STAT(n)=STAT*(n)+kΔS. In practice k istaken as equal to 1. In more specific terms, it is shown that therefining value ΔS increases in algebraic value when the gap between the“forward” and “backward” LPC filtering prediction gains increases, thefunction f_(r)(GPf, Gpb) being then an increasing function, whereas thisrefining value ΔS decreases in algebraic value when this sameaforementioned gap decreases, the aforementioned gap being definedbetween the prediction gain of the LPC “backward” filtering and theprediction gain of the LPC “forward” filtering. In fact, this functionis increasing or decreasing according to the definition of this gap.

Consequently, at the end of stage 1121 as shown in FIG. 2b, the value ofthe intermediate stationarity parameter STAT*(n) can then, for k=1, becorrected by the algebraic value of the aforementioned refining value ΔSin order to calculate the value of the stationarity parameter STAT(n).

Following stage 1121, the value of the stationarity parameter STAT(n) isset in stage 1122.

A more detailed description of the stage 1121 of FIG. 2b will be nowgiven in connection with FIG. 2c in a preferential version in whichseveral test criteria are applied as much as to the refining value as tothe values of the LPC “forward” and “backward” prediction gain in viewof optimizing the calculation process of the stationarity parameter.

As is shown in the aforementioned FIG. 2c, the stage 1121 can consist ofa first stage 1121 a enabling the calculation of the refining value ΔSfrom the previously quoted function f_(r)(Gpf, Gpb). Different examplesof useable functions will be given later in the description.

In the first place, the refining value ΔS is subject to a superioritycomparison test with the value 0, in a stage 1121 b, this comparisontest enabling in fact to determine the increase of this refining valueΔS.

On a positive response to the aforementioned test 1121 b, the refiningvalue ΔS being positive and corresponding to an increase in thecomparative value of the “forward” and “backward” LPC filteringprediction gains, the stage of increasing the value of the intermediatestationarity parameter from the refining value ΔS is moreover subjectedto a superiority condition of the gain value of “backward” LPCfiltering, in comparison with a first positive value determined in asuperiority comparison stage of the value of the “backward” LPCfiltering gain value Gpb in comparison with this first determinedpositive value, called S_(i).

On a negative response to the aforementioned test 1121 c, the value ofthe intermediate stationarity parameter STAT*(n) is attributed to thevalue of the stationarity parameter STAT(n) in a stage 1121 g.

On a positive response to the aforementioned test 1121 c, the increaseof the value of the intermediate stationarity parameter of the refiningvalue ΔS is furthermore subjected to an inferiority condition of thevalue of the intermediate stationarity parameter STAT*(n) in comparisonwith a second determined positive value STAT_(i) representing of coursea stationarity value. This inferiority test condition is carried out inthe stage 1121 e.

On a negative response to the aforementioned test 1121 e, the value ofthe intermediate stationarity parameter STAT*(n) in the aforementionedstage 1121 g is attributed to the value of the intermediate stationarityparameter STAT(n).

On a positive response to the inferiority test condition 1121 e thevalue of the intermediate stationarity parameter STAT*(n) increased bythe positive value ΔS of the refining value in the stage 1121 i isattributed to the value of the intermediate stationarity parameterSTAT(n).

In contrast, on a negative response to the aforementioned test 1121 b,the refining value ΔS being negative, the reduction stage of theintermediate stationarity parameter with the refining value ΔS, thisvalue being negative, is furthermore subject to an inferiority testcondition of the “backward” LPC filtering gain value Gpb in comparisonwith a determined third positive value called S_(d) in a comparisonstage 1121 d. This third determined positive value is of courserepresentative of an LPC filtering gain value.

On a negative response to the aforementioned test 1121 d the value ofthe intermediate stationarity parameter STAT*(n) is attributed to thevalue of the stationarity parameter STAT(n) in the stage 1121 g.

In contrast, on a positive response to the aforementioned test 1121 d,the reduction stage of the value of the intermediate stationarityparameter with the refining value ΔS is furthermore subject to asuperiority condition of the value of the intermediate stationarityparameter STAT*(n) in comparison with a fourth determined positivevalue, called STATd in a comparison test called 1121 f. Of course, thefourth determined positive value is representative of a chosenstationarity parameter value.

On a negative response to the aforementioned test 1121 f, the value ofthe intermediate stationarity parameter STAT*(n) is attributed to thestationarity parameter STAT(n) in the stage 1121 g.

On a positive response to the aforementioned test 1121 f, the value ofthe intermediate stationarity parameter STAT*(n) increased by thealgebraic value of the refining value ΔS, negative, is attributed to thestationarity parameter STAT(n), the value of the intermediatestationarity parameter being thus reduced in order to set up the valueof the stationarity parameter STAT(n) in the stage 1121 h.

At the end of the stages 1121 g, 1121 h and 1121 i, the stationarityparameter STAT(n) is thus set in the stage 1122 of FIG. 2b.

As regards the function f_(r)(Gpf, Gpb), it is shown that it may consistof a non linear function of the comparative value of the “forward” and“backward” LPC filtering gains in which the comparative value of the“forward” and “backward” LPC filtering prediction gains may themselvesconsist either in the ratio of, or in the difference of the “forward”and “backward” LPC filtering prediction gains. Other types of functions,such as linear functions, may be used.

A first example of the non linear function f_(r)(Gpf, Gpb) is shown inFIG. 2d.

In the version example of FIG. 2d, value pairs of the “backward” LPCfiltering prediction gain Gpb in the ordinate and the “forward” LPCfiltering gain Gpf enable allocating the positive refining values ΔS,ΔS>0 or negative ΔS<0 for a value of the ratio ρ=Gpb/Gpf correspondingto a respectively greater or lesser slope than that of the straight lineΔS=0.

In FIG. 2e, has been shown the case where the relative value of the“forward” and “backward” filtering prediction gains no longer correspondto the ratio of the gains ρ but to the difference of the aforementionedgains. In this case, the relative value of the “forward” and “backward”LPC filtering prediction gains can also be a non linear functionenabling allocating to the refining value ΔS for the values of thisdifference corresponding to the value pairs Gpb, Gpf corresponding tothe straight lines for which the abscissa origin is respectively less orgreater, in algebraic value, than the abscissa origin of the straightline ΔS=0. In the case of FIG. 2e, the straight lines delimiting thezones as a function of the sign of the refining value ΔS are parallel toeach other.

According to another particular aspect of the procedure which is theobject of the invention, it is recommended furthermore that it isaccepted not to adapt the stationarity index of the current block B_(n)during the silence frames, when for example the audio frequency signalis constituted by a speech signal comprising silences. In such a case,the stage 1111 of the stage 111 shown in FIG. 2b can be preceded by astage 1111 a consisting, for each successive current block, indetermining the mean energy of the audio frequency digital signal andcomparing in this same stage, on inferiority comparison criterion, thismean energy with a determined threshold value representative of asilence frame. In FIG. 2b, this threshold value is called ENER_SIL. On apositive response to the aforementioned test, the value of thestationarity parameter of the preceding block STAT(n−1) in theallocation stage 1111 b shown in FIG. 2b is attributed to the value ofthe stationarity parameter of the current block STAT(n). The stages 1111a and 1111 b are, in the aforementioned figure, shown as a dotted line,because it is reserved for example to the coding of a speech signal.

A more detailed description of the implementation decision functionD_(n) enabling the decision values d_(n)(n) to be obtained will be nowgiven in connection with FIG. 2f. This description is given in apreferential version in which this decision function, being able to becompared with that which is described in the previously mentionedarticle by the description, published by S. PROUST, C. LAMBLIN and D.MASSALOUX, is however temporally adapted, according to the object of thepresent invention in order to obtain the successive choice analysisd_(n)(n) values.

Starting with a stage 120, for the current block B_(n), in the firstplace a distance, called d_(LPC), between the LPC filter of the currentblock and that of the preceding block B_(n−1) is calculated. Thisdistance calculation is carried out for example by using the LSPfrequency parameters as previously mentioned in the description relatingto the procedure described in the aforementioned article.

It is noted:

the values of the thresholds S_PRED(n) and S_TRANS, S_STAT and G₁ beingreached in the criterion justified on the prediction gains of the“backward” and “forward” LPC filters;

the threshold values S_LSP_L and S_LSP_H being reached in the criterionjustified on the distances between LSP frequency vectors representingtwo “forward” LPC filters comparative to two consecutive blocks B_(n−1)and B_(n);

the prediction gain Gpf of the “forward” LPC filter;

the prediction gain Gpb of the “backward” filter; and

the prediction gain Gpi of the “forward” filter interpolated accordingto the method explained in the published article, previously mentionedin the description.

The criterion for establishing the decision function, in relation toFIG. 2f, is established in the manner below:

if the consecutive LPC filters are very stationary, i.e. ford_(LPC)<S_LSP_L, then, no switching of the “backward” LPC filtering withthe “forward” LPC filtering is carried out if it is in the “backward”LPC filtering mode, on condition that the prediction gain of the“backward” LPC filter is greater than the prediction gain of the“forward” LPC filter reduced by a S_STAT value. It is mentioned that theS_STAT value is chosen so as to favor the choice of a “backward” LPCfilter in the presence of a large stationarity of the spectrum measuredby means of the distance d_(LPC);

if the consecutive LPC filters have a significant transition, i.e. ford_(LPC)>S_LSP_H and if Gpf>Gpb−S_TRANS, then the chosen filtering modeis the “forward” LPC filtering, i.e. d_(n)(n)=0, symbolic value “fwd”,otherwise, d_(n)(n) is almost equal to 1, symbolic value “bwd”. It ismentioned that the value of S_TRANS is chosen so as to strongly favorthe choice of the “forward” LPC filter in the presence of a spectrumtransition measured by means of the distance d_(LPC);

otherwise, in all other cases, if Gpb>Gpf-S_PRED and Gpi>Gpf-S_PRED,then, the LCP filter retained is the interpolated “backward” LPC filter,on condition that the gain of this latter and that of the pure“backward” LPC filter exceeds the threshold value G_(i) previouslymentioned. If the condition on the values of the aforementionedprediction gain is not fulfilled, then, the “forward” LPC filtering ischosen.

In order to increase the number of transmitted “forward” LPC filters andthus to increase the strength of the coding system to the transmissionerrors, the “forward” LPC filtering mode may be chosen with advantage assoon as the energy signal to be coded E_(n), i.e. the energy of thecorresponding block B_(n), becomes less than the value of the energy ofa silence frame ENER_SIL, this value of energy corresponding to theminimum audible level.

The set of the conditions enabling the establishment of the decisionfunction D_(n) and the obtaining of the corresponding chosen analysisvalues d_(n)(n), is illustrated in FIG. 2f with temporal adaptation ofthe decision function D_(n).

The value of the stationarity parameter STAT(n) can for example belocated on a scale of 0, corresponding to the non-stationary STAT_(n)value, to 100, corresponding to the very stationary STAT_((n)) value.

According to the value of the stationarity parameter STAT(n), thedecision function D_(n) is modified by adaptation of the value of thethresholds.

The more the stationarity of the signal increases, the more the“backward” LPC filtering mode is favored: the thresholds S_PRED, S_LSPand S_LSP_H are increased.

As a non-limited example, the modification functions for each currentLPC block B_(n) of the aforementioned threshold values have been shown:

S_PRED(n)=f_(s) _(—) _(PRED)(STAT(n)) with the function f_(s) _(—)_(PRED) increasing with the value of STAT(N);

S_LSP_L(n)=f_(S) _(—) _(LPC) _(—) _(L)(STAT(n)) with the function f_(S)_(—) _(LPC) _(—) _(L) increasing;

S_LSP_L(n)=f_(s) _(—) _(LPC) _(—) _(R)(STAT(n)) with the function f_(s)_(—) _(LPC) _(—) _(R) increasing.

In the adaptation of the aforementioned threshold values, it has beenshown that the increasing functions mentioned are for example functionsfor that which concerns the functions f_(S) _(—) _(LPC) _(—) _(L) andf_(S) _(—) _(LPC) _(—) _(H). The function f_(S) _(—) _(PRED) is arefined function of the variable stationarity parameter, of the form:

S_PRED(n)=α.STAT(n)+β

where α and β are two real values between 0 and 1 and where the value ofS_PRED(n) is limited in the interval [S_PRED_(m), S_PRED_(M)],S_PRED_(m) and S_PRED_(M) representing two experimentally determinedvalues.

In order to limit yet again the risk of switching filters, it is thenpossible to choose, when the stationarity parameter STAT(n) is less thana given threshold value S_(FWD), to require the “forward” LPC filteringmode.

On the other hand, the S_TRANS, S_STAT and G₁ threshold values retain afixed value, these values being able for example to be equal to −1 dB, 5dB and 0 dB respectively.

The establishment of the decision function D_(n) and the obtaining ofthe analysis choice values d_(n)(n) are illustrated in the following wayin FIG. 2f: following the aforementioned stage 120, carrying out a teststage 121 relative to the energy of the current LPC block B_(n), by aninferiority comparison with the silence energy value ENER_SIL or withthe value of the stationarity parameter STAT(n), compared by aninferiority comparison with the value S_(FWD) quoted previously in thedescription. On a positive response to the aforementioned test 121, thechoice analysis value d_(n)(n) is taken as equal to 0, i.e. a symbolicvalue “fwd” in the stage 122.

On a negative response to the aforementioned test 121, a new test iscarried out relative to the choice analysis value d_(n−1)(n−1) with thelogical value 1, i.e. with the symbolic value “bwd”.

On a positive response to the aforementioned test 123, a new test iscarried out on the aforementioned LPC filtering distance d_(LPC), in astage 124, in comparison with the threshold value S_LSP_H(n) bysuperiority comparison with this threshold value.

On a positive response to the aforementioned test 124, a new test 126 ais carried out, consisting of comparing the “forward LPC filteringprediction gain, Gpf, with the “backward” LPC filtering prediction gain,Gpb, reduced by the threshold value S_TRANS.

On a positive response to the aforementioned test 126 a, the logicalvalue 0, symbolic value “fwd”, is attributed to the choice analysisvalue d_(n)(n), and on a negative response to the aforementioned test126 a, the same value of choice analysis is attributed the value 1,symbolic value “bwd”. The corresponding stages are called 128 and 129.

On a negative response to the previously mentioned test 124, a new test125 is carried out. The test 125 consists in carrying out a comparisonof the distance of the LPC filtering, d_(LPC), by inferiority comparisonwith the threshold value S_LSP_L(n).

On a positive response to the test 125, a new test 126 b is carried outby superiority comparison of the “backward” LPC filtering predictiongain with the “forward” LPC filtering prediction gain reduced by thepreviously mentioned value S_STAT.

On a positive response to the test 126 b, the logical value 1 isattributed to the value of the choice analysis d_(n)(n) in the stage129, i.e. the symbolic value “bwd”.

On a negative response to the test 126 b, the logical value 0 isattributed to the value of the choice analysis d_(n)(n), i.e. thesymbolic value “fwd”, stage 128.

In contrast, on a negative response to the test 125, a new test iscarried out, in a stage 127, this test consisting of verifying thecomparison conditions of the “backward” LPC filtering gain Gpb with the“forward” LPC filtering prediction gain reduced by the threshold valueS_PRED(n), by superiority comparison of the intermediate LPC filteringprediction gain Gpi with the “forward” LPC filtering prediction gainvalue reduced by the aforementioned threshold value S_PRED(n) and bysuperiority comparison of the “backward” filtering prediction gain Gpbwith the threshold value G₁, as well as comparison of the value of theintermediate filtering prediction gain Gpi with the threshold value G₁.

It is mentioned that the negative response to the test 123 previouslymentioned in the description leads also to the carrying out of theaforementioned test 127.

On a positive response to the previously mentioned test 127, the logicalvalue 1 is attributed to the value of the choice analysis d_(n)(n), i.e.the symbolic value “bwd” in the stage 129, whereas with a negativeresponse to the aforementioned test 127, the logical value 0 is on thecontrary attributed to the value of the choice analysis d_(n)(n), i.e.the symbolic value “fwd” in the stage 128.

Thus is set, by means of using the decision function D_(n), the value ofthe choice analysis d_(n)(n) obtained with the aforementioned logicalvalues 1 or 0, these logical values being however connected to apriority or absence of priority value of returning to the “backward” or“forward” filtering mode as a function of the value of the stationarityparameter.

A more detailed description of a coding device of an audio frequencydigital signal by double analysis on the criterion of respectively“forward” or “backward” LPC choice analysis in a transmitted codedsignal, according to the object of the present invention, will now begiven in connection with FIG. 3.

In a practical manner, it is mentioned that the digital signal to becoded is subdivided into frames constituted by successive blocks ofsamples, each block comprising a given number N of samples for example.

In FIG. 3, constitution mode of the audio frequency digital signal to becoded in successive blocks of samples B_(n) has not been shown for thisoperating mode is well known in the state of the technical art and canbe carried out form a simple memory buffer, for example addressed toperiodically read the frame frequency and the block frequency.

As shown furthermore in the aforementioned FIG. 3, the coding devicewhich is the object of this invention includes a “forward” LPC analysisfilter, carrying the reference 1A, and a “backward” analysis filter,carrying the reference 1B, in order to enable the delivery of atransmitted coded signal consisting of LPC filtering parametersaccompanied by an analysis decision indication, as well as Prparameters, relative to the harmonic analysis and to the excitationsignal CELP.

Generally, it is shown that the analysis decision indication correspondsof the value of choice analysis d_(n)(n) as mentioned previously in thedescription. In so far as the LPC filtering parameters, it is mentionedthat these correspond to specific parameters, according to the mode usedof the coding method which is the object of the present invention aswill be described later in the description.

In FIG. 3, also has been shown, in the coding device according to theinvention, the existence of an adaptive filter operating as a functionof the value of the stationarity parameter, this adaptive filtercarrying the reference 1E. This adaptive filter 1E receives, it isunderstood of course, the original digital signal called s_(n(t)), i.e.the current block B_(n). The filter 1E uses the filtering LPC parameterin order to calculate the residual signal which in turn is coded by themodule 1F. These LPC parameters, as well as the filtering decisionindication constitute a part of the coded signal which is transmitted tothe decoder.

Furthermore, as shown in FIG. 3, the coding device which is the objectof the present invention includes a coding means, carrying the reference1F, of a non transmitted residue coding signal, the residue codingsignal, designated by res_(n(t)) is directly available at the output ofthe adaptive filter 1E, this signal being thus delivered to the inputwith the audio frequency digital signal at the coding module of the nottransmitted residue coding signal, in order to generate a synthesisresidue signal, res_syn_(n(t)).

A reverse filtering module, carrying the reference 1G, receives thesynthesis residue signal and enables the delivery of a synthesis signalreferenced s_syn_(n(t)).

A memorization module 1H receives the aforementioned synthesis signals_syn_(n(t)) in order to deliver the aforementioned synthesis signal forthe previous block to the current block B_(n), the synthesis signal thusobtained being designated by s_syn_(n−1)(t). This synthesis signal isdelivered to the “backward” LPC analysis filter carrying the reference1B in the aforementioned FIG. 3.

The coding device, which is the object of the present invention, asshown in FIG. 3, enables carrying out a coding of the audio frequencydigital signal on the aforementioned audio frequency digital signal fromthe “forward” LPC filter for the non-stationary zones and on theaforementioned synthesis signal s_syn_(n−1)(t) from the “backward” LPCfilter 1B for the stationary zones, as will be described below.

As will be observed in the aforementioned FIG. 3, the device which isthe object of the invention comprises in this aim, for each current LPCblock B_(n), a calculation module 1C of the degree of stationarity ofthe audio frequency digital signal according to a stationarity parameterthe value of which is between a maximum stationarity value and a minimumstationarity value. Of course, the stationarity parameter is theparameter STAT(n) previously described in the description according tothe coding procedure which is the object of the present invention. Themaximum and minimum stationarity values are also defined previously.

As has been shown furthermore in FIG. 3, the coding device which is theobject of the invention includes a module, called 1D, for establishingfrom the aforementioned stationarity parameter STAT(n) a decisionfunction and an LPC choice analysis value, the decision function beingcalled D_(n) as previously mentioned in the description, and the LPCchoice analysis value being of course corresponding to the value of theLPC choice analysis called d_(n)(n) previously mentioned in thedescription. It will be recalled that the value of the choice analysisd_(n)(n) can take the values 0 or 1, logical values, which correspond tothe choice analysis symbolic value “fwd” and “bwd” for the “forward” andbackward” LPC analysis respectively.

It is understood in particular that concerning the establishment of thedecision function D_(n), which corresponds for example to a softwareimplementation, such as previously described in connection with FIG. 2f.Furthermore, the coding device according to the invention such as shownin FIG. 3 includes an LPC filtering analysis discrimination module,called 1D₂, this module receiving the value of the choice analysisd_(n)(n) and enabling delivering, for the current LPC block B_(n), thevalue of the LPC “backward” and “forward” filtering parametersrespectively as a function of the aforementioned value of choiceanalysis. It is clearly understood that the “backward” LPC filteringanalysis as well as the “forward” LPC filtering analysis parameters areof course available in digital form at the filters carrying thereference 1B and 1A respectively in FIG. 3. These parameters aredesignated respectively Af_(n)(z) for the “forward” LPC filteringanalysis parameters with regard to the “forward” LPC analysis filter,carrying the reference 1A, and by Ab_(n)(z) for the “backward” LPCanalysis parameters with regard to the “backward” LPC analysis filtercarrying the reference 1B. These parameters are delivered to the module1D₁ and the module 1D₂ respectively.

As regards the creation of the equipment of the discrimination module1D₁, it is shown that it may for example, in a non-limitative version,consist of two distinct memory zones enabling the memorization of thefiltering parameters Af_(n)(z) and Ab_(n)(z) respectively, the value ofchoice analysis d_(n)(n) as a function of its current logical value, 0or 1, enabling the addressing for reading the values of the memorizedfiltering parameters by the module 1D₂ for example and the transmissionof these filtering parameters by this latter.

Finally, as shown in FIG. 3, it has been shown that the coding deviceaccording to the object of the present invention, for the operation ofthe adaptive filter according to the stationarity value carrying thereference 1E , can be carried out by a filtering element the transferfunction of which, called A(z), is established from the filteringparameter values delivered by the discrimination module 1D₂ previouslymentioned.

It is understood as well that the adaptive filtering module 1E can beachieved by a filter with adjustable coefficients, with the value of thecoefficients of this latter delivered by the discrimination module 1D₂previously mentioned. The filtering carried out by the module 1E is thusof the adaptive type operating as a function of the degree ofstationarity of the audio frequency digital signal to be coded. Themodule 1E thus delivers, from the original audio frequency digitalsignal sn_((t)), the LPC filtering residue signal designated byres_(n)(t) to the coding module of the residue 1F, which enables thenthe delivery of the LPC synthesis residue signal designated byres_syn_(n)(t).

Finally, the module 1 G is a filtering module the transfer function ofwhich is the reverse of the transfer function of the module 1E obtainedform the memorized parameters of this latter. It receives the LPCsynthesis residue signal res_syn_(n)(t) delivered by the coding moduleof the coding residue delivered by the module 1F. It is thus understoodthat the coding of the audio frequency digital signal s_(n)(t) iscarried out in the module 1E through the LPC “forward” and “backward”analysis respectively which is carried out by the LPC “forward” and“backward” analysis filters 1A, 1B, the coded signal s_c_(n)(t)consisting in the transmission of the “forward” LPC filtering parameterswhen the value of the choice analysis d_(n)(n) has the symbolic value“fwd” as well as the indication of the choice analysis, i.e. of thevalue of the preceding quoted value of the choice analysis. This mode ofoperation enables carrying out the coding of the audio frequency digitalsignal and favoring holding it in one of the respectively “forward” and“backward” LPC filtering modes, as a function of the degree ofstationarity of the digital signal, and limiting furthermore the numberof switchings from one to the other of the considered filtering modes.

A decoding device of a coded audio frequency digital signal by doubleanalysis on the criterion of respectively “forward” and “backward” LPCanalysis, to a coded signal transmitted according to the coding methodwhich is the object of the present invention, and by means of using acoding device such as shown in FIG. 3 for example, will now be describedin connection with FIG. 4.

In a general manner, it is shown that the transmitted coded signals_c_(n)(t) consists for each LPC analysis block of the value of theaforementioned choice analysis and, in the case where the value ofchoice analysis corresponds for the considered LPC analysis block to a“forward” LPC analysis, of the “forward” LPC filtering parameters aswell as the coding parameters of the LPC filtering residue, Pr_(n)parameters, i.e. of the signal res_(n)(t) in a synthesis residue signalres_syn_(n)(t) by the residue coding module 1F.

As shown in FIG. 4, it is shown that the decoding device comprises atleast a synthesis module, referenced 2A, of the filtering residue signalreceiving the coding parameters of the LPC residue delivered by themodule 1F. The module 2A decodes the coding parameters supplied by themodule 1F and delivers consequently a synthesis residue signal, which isreferenced in FIG. 4 res_syn_(n)(t).

The decoding device as shown in FIG. 4 comprises also a module, carryingthe reference 2B, of reverse adaptive filtering as a function of thedegree of stationarity, receiving the previously quoted synthesisresidue signal, delivered by the module 2A, and enabling the generationof a synthesis signal s_syn_(n)(t) representative of the audio frequencydigital signal, this signal constituting in fact the decoded signal.

It is of course understood that the reverse filtering module 2B uses thefiltering parameters received by the decoder due to the fact of thetransmission, are the “forward” LPC analysis parameters when these aretransmitted and that the analysis decision corresponds to a “forward”LPC analysis or, in contrast, the “backward” filtering analysisparameters as will be described below.

With this aim, the decoding device which is the object of the presentinvention comprises of course a “backward” LPC filtering module,carrying the reference 2D, receiving the synthesis signal, i.e. thesignal referenced s_syn_(n)(t) for the LPC block preceding the currentLPC block, this synthesis signal being thus referenced s s_syn_(n−1)(t)in FIG. 4. With this aim, it is understood that the synthesis signalrelative to the current block B_(n) and referenced s_syn_(n)(t) may thenbe delivered to the “backward” filtering module 2D by means of amemorization module, carrying the reference 2E, enabling in fact, by anadapted addressing for reading, to shift the reading of the synthesissignal to that corresponding to the block preceding, the current blockB_(n).

Finally, and to ensure the aforementioned operating mode, the decodingdevice which is the object of the present invention, as shown in FIG. 4,further includes a discriminator module carrying the reference 2C,enabling the carrying out of a “forward” and “backward” LPCdiscrimination analysis respectively. The module 2C receives, on the onehand, to control the discrimination, the value of choice analysisreceived, i.e. the value d_(n)(n), and, on the other hand, the “forward”LPC filtering parameters, i.e. the parameters Af_(n)(z) transmitted, aswell as the “backward” LPC filtering parameters Ab_(n)(z) obtained bymeans of the module 2D. The module 2C thus enables delivering, as afunction of the choice analysis value, i.e. of the value d_(n)(n),either the “forward” filtering parameters Af_(n)(z), or the “backward”filtering parameters Ab_(n)(z) to the reverse adaptive filtering module2B as a function of the degree of stationarity.

As regards the material embodiment of the modules 2C and 2B, it ismentioned that these may simply consist of modules approximatelyidentical to the modules 1D₂ and 1E or, more particularly, 1G of FIG. 3.

As regards the effective embodiment of a coding device according to theobject of the present invention, enabling using the procedure such asdescribed previously in the description, two specific versions have beencarried out.

Telephonic Band CELP Type Encoder, According to High Output Extension ofthe UIT-T Standard at 8 kb/s:

The actual encoder consisted of a telephonic band encoder from 300 to3400 Hz, with an output of 12 kb/s of CELP type. The frames wereconstituted over a duration of 10 ms for an excitation supplied byalgebraic codepages according to the technique called ACELP previouslymentioned in the description.

The “forward” LPC analysis was an analysis of order 10 and the“backward” LPC analysis an analysis of order 30 every 80 samples.

A separation for the coding of the residue into two sub-blocks of 40samples has been carried out. Each block B_(n) included 80 samples.

Adaptation of the Stationarity Parameter STAT(n)

The aforementioned stationarity parameter varies between two extremevalues 0 and 100, the aforementioned values STAT_(m) and STAT_(M).

The adaptation functions previously described in the description, and inparticular the functions f_(a)(N_BWD) and f_(b)(N_BWD) were such that:${f_{a}\left( {N_{—}{BWD}} \right)} = \left\{ \begin{matrix}{{1.56\quad {if}\quad N_{—}{BWD}} > 20} \\{{7.81\quad {if}\quad N_{—}{BWD}} = 20} \\{0\quad {otherwise}}\end{matrix} \right.$

${f_{b}\left( {N_{—}{BWD}} \right)} = \left\{ \begin{matrix}{{{0.78 \cdot \left( {20 - {N_{—}{BWD}}} \right)}\quad {if}\quad N_{—}{BWD}} \leq 20} \\{0\quad {otherwise}}\end{matrix} \right.$

In these relations, x=DGfb.

As regards the function f_(r) enabling the refining value ΔS previouslymentioned in the description to be established, this is a step functionof the variable x, with x=Gpb−Gpf and ΔS=f_(r)(x) and having the value:${f_{r}(x)} = \left\{ \begin{matrix}{{10\quad {if}\quad x} \geq 4} \\{{7.5\quad {if}\quad x} \in \left\lbrack {3;{4\lbrack}} \right.} \\{{5\quad {if}\quad x} \in \left\lbrack {2;{3\lbrack}} \right.} \\{{2.5\quad {if}\quad x} \in \left\lbrack {1;{2\lbrack}} \right.} \\{{1.25\quad {if}\quad x} \in \left\lbrack {0;{1\lbrack}} \right.} \\{{0.625\quad {if}\quad x} \in \left\lbrack {{- 1};{0\lbrack}} \right.} \\{{0\quad {if}\quad x} \in \left\lbrack {{- 2};{- {1\lbrack}}} \right.} \\{{{- 2.5}\quad {if}\quad x} \in \left\lbrack {{- 3};{- {2\lbrack}}} \right.} \\{{{- 5}\quad {if}\quad x} \in \left\lbrack {{- 4};{- {3\lbrack}}} \right.} \\{{{- 10}\quad {if}\quad x} \in \left\lbrack {{- 5};{- {4\lbrack}}} \right.} \\{{{- 20}\quad {if}\quad x} < {- 5}}\end{matrix} \right.$

The tuning of STAT(n) is furthermore subject to the following conditionspreviously mentioned in relation with FIG. 2c:

If ΔS>0:

If STAT*(n)<STAT_(i) STAT(n)=STAT*(n)+ΔS

Otherwise STAT(n)=STAT*(n)

Otherwise:

STAT(n)=STAT*(n)

with STAT_(i)=40.6

The other test conditions referenced 1121 d, 1121 c and 1121 f in FIG.2c have not been used in the version.

Adaptation of the Decision Thresholds

As regards the decision thresholds:

S_PRED is adapted in the following manner:

S_PRED(n)=0.03.STAT(n)+1.0

S_PRED ε[S_PRED_(m), S_PRED_(M)], S_PRED_(m)=1.03 and S_PRED_(M)=4;

The threshold S_LSP_L is adapted using the following step function:${S_{—}{LPS}_{—}{L(n)}} = {{f_{S_{-}{LPC}_{-}L}\left( {{STAT}(n)} \right)} = \left\{ \begin{matrix}{{0.015\quad {if}\quad {{STAT}(n)}} = 100} \\{0\quad {otherwise}}\end{matrix} \right.}$

The threshold value S_STAT used in case of stationarity of the LPCfilters measured using the threshold S_LSP_L has been fixed at 4.0 dB.

The threshold S_LSP_H has not been used in this version.

The value of the threshold G₁ has been fixed at 0 db.

As regards the energy value characterizing a silence frame ENER_SIL,this value has been fixed at 40 dB measured over the 80 samples s(i) ofthe current block B_(n):${{ENER}_{—}{SIL}} = {10 \cdot {{Log}\left( {\sum\limits_{i = 0}^{i(80}\left( {s(i)} \right)^{2}} \right)}}$

As regards the value of the previously mentioned S_(FWD) thresholdintended to limit still further the risk of switching by imposing the“forward” LPC filter mode when the STAT(n) value is lower than thisthreshold, this S_(FWD) value has been set at 40.6.

A second Version of a CELP Broadened Band Encoder with Two Sub-bands16/24/32 kb/s was Carried Out in the Following Conditions:

a broadened band encoder of 0 to 7000 Hz in two sub-bands. A main bandwas encoded with the CELP technique, frame with 120 samples, excitationcreated by algebraic codepages, and transmission of certain energy andspectrum characteristics of a host band of between 6000 Hz and 7000 Hz.

“forward” LPC analysis with 14 coefficients and “backward” LPC analysiswith 50 coefficients every 120 samples. In “forward” LPC analysis mode,separation into two 60 sample LPC sub-blocks, the filter used for thefirst sub-block being interpolated from the current filter and theprevious filter.

Calculation of the Stationarity Parameter STAT(n)

In this version, the aforementioned stationarity parameter variesbetween the two extreme values 0 and 120, the aforementioned STAT_(m)and STAT_(M) values.

As regards the adaptation of the stationarity parameter value STAT(n),the values of the f_(a)(N_BWD) and f_(b)(N_BWD) functions are such that:${f_{a}\left( {N_{—}{BWD}} \right)} = \left\{ {{\begin{matrix}{{4\quad {if}\quad N_{—}{BWD}} > 10} \\{{20\quad {if}\quad N_{—}{BWD}} = 10} \\{0\quad {otherwise}}\end{matrix}{f_{b}\left( {N_{—}{BWD}} \right)}} = \left\{ \begin{matrix}{{10 - {N_{—}{BWD}\quad {if}\quad N_{—}{BWD}}} \leq 10} \\{0\quad {{otherwise}.}}\end{matrix} \right.} \right.$

As regards the fr function allowing the refining value ΔS previouslymentioned in the description to be set, this is a step function of thevariable x, with x=Gpb/Gpf and ΔS=f_(r)(x) and having a value of:${f_{r}(x)} = \left\{ \begin{matrix}{{9\quad {if}\quad x} \geq 1.2} \\{{6\quad {if}\quad x} \in \left\lbrack {1.1;{1.2\lbrack}} \right.} \\{{3\quad {if}\quad x} \in \left\lbrack {1.05;{1.1\lbrack}} \right.} \\{{1.5\quad {if}\quad x} \in \left\lbrack {1.0;{1.05\lbrack}} \right.} \\{{0.75\quad {if}\quad x} \in \left\lbrack {0.95;{1.0\lbrack}} \right.} \\{{0\quad {if}\quad x} \in \left\lbrack {0.9;{0.95\lbrack}} \right.} \\{{{- 1.5}\quad {if}\quad x} \in \left\lbrack {0.85;{0.9\lbrack}} \right.} \\{{{- 3}\quad {if}\quad x} \in \left\lbrack {0.8;{0.85\lbrack}} \right.} \\{{{- 6}\quad {if}\quad x} \in \left\lbrack {0.75;{0.8\lbrack}} \right.} \\{{{- 12}\quad {if}\quad x} < 0.75}\end{matrix} \right.$

The tuning of STAT(n) is moreover subject to the following previouslymentioned conditions in relation with FIG. 2c: If ΔS>0:

If Gpb > S_(i) If STAT*(n) < STAT_(i) STAT(n) = STAT*(n) + ΔS OtherwiseSTAT(n) = STAT*(n) Otherwise STAT(n) = STAT*(n) Otherwise: If STAT*(n) <STAT_(i) STAT(n) = STAT*(n) + ΔS Otherwise STAT(n) = STAT*(n) withSTAT_(i) = 80, S_(i) = 0 dB.

The other test conditions referenced 1121 h and 1121 d in FIG. 2c havenot been used in this version.

Adaptation of Decision Thresholds

As regards the decision thresholds:

S_PRED is adapted in the following way:

S_PRED(n)=0.03 STAT(n)−0.5 limited in the interval

[S_PRED_(m), S_PREDM ]

 with S_PRED_(m)=0.5 and S_PRED_(M)=2.5.

The S_LSP_L threshold is adapted with the help of the following stepfunction:${S_{—}{LSP}_{—}{L(n)}} = {{f_{S_{-}{LSP}_{-}L}\left( {{STAT}(n)} \right)} = \left\{ \begin{matrix}{{0.02\quad {if}\quad {{STAT}(n)}} > 100} \\{0.01\quad {otherwise}}\end{matrix} \right.}$

The S_LSP_H threshold is adapted with the help of the following stepfunction:${S_{—}{LSP}_{—}{H(n)}} = {{f_{S_{-}{LSP}_{-}L}\left( {{STAT}(n)} \right)} = \left\{ \begin{matrix}{{0.08\quad {if}\quad {{STAT}(n)}} > 100} \\{0.01\quad {otherwise}}\end{matrix} \right.}$

The value of the S_TRANS threshold used in the case of transition of theLPC filters measured with the help of the S_LSP_H threshold has been setat 0 dB.

The value of the S_STAT threshold used in the case of stationarity ofthe LPC filters measured with the help of the S_LSP_L threshold has beenset at 2.5 dB.

The value of the G threshold has been set at 0 dB.

As regards the energy value characterizing a frame of silence ENER_SIL,this value has been set at 50 dB measured over the 120 samples s(i) ofthe current block B_(n):${{ENER}_{—}{SIL}} = {{Log}\left( {\sum\limits_{i = 0}^{i < 120}\left( {s(i)} \right)^{2}} \right)}$

As regards the value of the previously mentioned S_(FWD) thresholdintended to limit still further the risk of switching by imposing the“forward” LPC filter mode when the STAT(n) value is lower than thisthreshold, this S_(FWD) value has been set at 60.

What is claimed is:
 1. A method for encoding a digital audio signal bydual analysis according to a choice criterion of LPC “forward” and“backward” analysis respectively into a transmitted encoded signalconsisting of LPC filtering parameters accompanied by analysis decisioninformation, and into a residue encoding signal, not transmitted, saiddigital audio signal being subdivided into frames, a succession ofblocks of a specified number of samples, the encoding of said digitalaudio signal being carried out on this signal through a “forward” LPCfiltering for non-stationary zones respectively on a synthesis signal,obtained from said residue encoding signal, through a “backward” LPCfiltering for stationary zones, wherein said choice criterion consists,on each current block of said succession of current blocks constitutinga current frame: in defining the degree of stationarity of the digitalaudio signal according to a stationarity parameter, the value of whichlies between a maximum stationarity value and a minimum stationarityvalue; in establishing, from said stationarity parameter, an analysischoice value, from a decision function; in applying said analysis choicevalue to the “forward” LPC filtering so as to carry out the encoding ofsaid digital audio signal by “forward” LPC filtering for non-stationaryzones on said digital audio signal, and by “backward” LPC filteringrespectively for stationary zones on said synthesis signal, which makesit possible to favor the maintenance of the digital audio signal in oneof the “forward” and “backward” filtering modes respectively in relationto the degree of stationarity and to limit the amount of switching fromone to the other and vice versa of the filtering modes.
 2. The methodaccording to claim 1, wherein said decision function is an adaptivefunction, actualized for each current block from the stationarityparameter, said actualization of said adaptive function making itpossible to favor the maintenance of the digital audio signal filteringin one of the “forward” and “backward” filtering modes respectively as afunction of the degree of stationarity of said digital audio signal andthus to limit the amount of switching from one to the other and viceversa of the filtering modes.
 3. The method according to claim 1,wherein said analysis choice value established from said decisionfunction corresponds to a “forward” LPC filtering mode priority valueand a “backward” LPC filtering mode priority value respectively.
 4. Themethod according to claim 1, wherein the stage consisting in specifyingthe degree of stationarity of each current block of said digital audiosignal consists, starting from an arbitrary starting value of saidstationarity parameter: in calculating for said current block anintermediate stationarity parameter value, as a function of a specifiednumber of analysis choice values, obtained for different successiveblocks prior to said current block of said succession of blocks, and ofthe stationarity parameter value of the block preceding the said currentblock; in tuning said intermediate stationarity parameter value as afunction of the value of prediction gains of the “forward” and“backward” LPC filtering of the frame preceding said current frame. 5.The method according to claim 4, wherein the stage consisting, for eachcurrent block, in calculating an intermediate stationarity parametervalue consists: in discriminating between the “forward” LPC or“backward” LPC analysis mode of the block preceding said current block;and for any previous block analyzed by “backward” LPC analysis mode: inspecifying the number of previous frames consecutively analyzed in“backward” LPC analysis mode, in comparing, according to a superioritycomparison criterion, said number of previous frames with a firstarbitrary value representative of a number of successive frames analyzedin “backward” LPC mode, and on positive response to this superioritycomparison, attributing to said intermediate stationarity parametervalue the stationarity parameter value of the block preceding saidcurrent block, augmented by a specified value function of said firstarbitrary value, and on negative response to this superioritycomparison, attributing to said intermediate stationarity parametervalue the stationarity parameter value of the block preceding saidcurrent block; and for any previous block analyzed in “forward” LPCanalysis mode, in specifying according to a test criterion theoccurrence of a transition from “backward” LPC analysis mode to“forward” LPC analysis mode between the block prior to said precedingblock and said preceding block, and on positive response to said test ofoccurrence, in comparing, according to an inferiority comparisoncriterion, said number of previous frames with a second arbitrary valuerepresentative of a number of successive frames analyzed in “backward”LPC mode preceding said preceding block, and on positive response tosaid inferiority comparison, attributing to said intermediatestationarity parameter value the stationarity parameter value of saidblock preceding the current block, reduced by a specified value which isa function of said second arbitrary value, and on negative response tosaid inferiority comparison, attributing to said intermediatestationarity parameter value the stationarity parameter value of saidpreceding block.
 6. The method according to claim 4, wherein the stageconsisting for each current block in tuning said intermediatestationarity parameter value consists: in distinguishing betweenprediction gains of the “forward” LPC filtering and “backward” LPCfiltering; in modifying the intermediate stationarity parameter value ofa refining value function of the relative value of prediction gains of“forward” and “backward” LPC filtering, the modification, increase orreduction, of the intermediate stationarity parameter value beingproportional to said refining value.
 7. The method according to claim 6,wherein the stage of increase proportional to said refining value of theintermediate stationarity parameter value is moreover subject to acondition of superiority of said value of “backward” LPC filtering gainrelative to a first specified positive value and to a condition ofinferiority of the value of said intermediate stationarity parametervalue relative to a second specified positive value.
 8. The methodaccording to claim 6, wherein the stage of reduction proportional tosaid refining value of the intermediate stationarity parameter value ismoreover subject to a condition of inferiority of said value of“backward” LPC filtering gain relative to a third specified positivevalue and to a condition of superiority of the value of saidintermediate stationarity parameter value relative to a fourth specifiedpositive value.
 9. The method according to claim 6, wherein saidrelative value of the prediction gains of “forward” and “backward” LPCfiltering consists in the ratio or the difference between predictiongains of “forward” and “backward” LPC filtering.
 10. The methodaccording to claim 1, wherein said method consists in addition, for eachsuccessive current block: in establishing the average energy of saiddigital audio signal, in comparing, according to an inferioritycomparison criterion, said average energy with a specified thresholdvalue representative of a silence frame, and on positive response tosaid inferiority comparison, in attributing to said stationarityparameter of the current block the stationarity parameter value of thepreceding block.
 11. The method according to claim 2, wherein, for adegree of stationarity represented by a stationarity parameter between aminimum value and a maximum value, said minimum value representing thedegree of stationarity of a substantially non-stationary digital signaland said maximum value representing the degree of stationarity of asubstantially stationary signal, said adaptive function constituting thedecision function is an increasing function of the priority value of the“backward” LPC filtering mode according to the increasing degree ofstationarity of said digital signal.
 12. An encoding device for adigital audio signal by dual analysis according to a choice criterion of“forward” and “backward” LPC analysis respectively into a transmittedencoded signal, said digital signal being subdivided into framesconstituted by successive blocks comprising a specified number ofsamples, said encoding device comprising a “forward” LPC analysis filterand a “backward” LPC filter enabling delivery of a transmitted encodedsignal consisting of LPC filtering parameters accompanied by an analysisdecision indication and a means of encoding an encoding residue signal,not transmitted, enabling generation of a synthesis residue signal, theencoding of said digital audio signal being carried out on this digitalaudio signal from the “forward” LPC filter for non-stationary zones andon this synthesis signal, from the “backward” LPC filter respectivelyfor stationary zones, wherein said encoding device comprises inaddition, for each current LPC block; calculation means of the degree ofstationarity of said digital audio signal, according to a stationarityparameter the value of which is between a minimum stationarity value anda maximum stationarity value; setting means, from a stationarityparameter, of a decision function enabling an LPC analysis choice valueto be set; discrimination means of LPC analysis receiving said analysischoice value and enabling delivery, for said LPC current block, of thevalue of the “backward” and “forward” LPC filtering parametersrespectively as a function of said analysis choice value; adaptivefiltering means as a function of the degree of stationarity receivingsaid digital audio signal and the value of the “forward” and “backward”LPC filtering parameters respectively as a function of said analysischoice value and delivering the encoding residue signal to said encodingmeans of the encoding residue signal, which makes it possible to encodesaid digital audio signal and to favor the maintenance of said digitalaudio signal in one of the “forward” and “backward” filtering modesrespectively in relation to the degree of stationarity of said digitalsignal and to limit the amount of switching from one to the other andvice versa of the filtering modes.
 13. The encoding device according toclaim 12, wherein said transmitted encoded signal consists, for each LPCanalysis block, of: said analysis value, and in the case where theanalysis choice value corresponds for LPC analysis block considered, toa “forward” LPC analysis; said “forward” PC filtering parameters.
 14. Adecoding device of a digital audio signal encoded by dual analysisaccording to a choice criterion of “forward” and “backward” LPC analysisrespectively, into a transmitted encoded signal consisting of LPCfiltering parameters accompanied by an analysis decision indication,wherein that said transmitted encoded signal, consisting for each LPCanalysis block of said analysis choice value and corresponding for theLPC analysis block considered to “forward” LPC analysis in “forward” LPCfiltering parameters, said decoding device comprises at least: synthesismeans for the filtering residue signal receiving said encodingparameters of the LPC residue and delivering a synthesis residue signal,reverse filtering adaptive means as a function of the degree ofstationarity, receiving the synthesis residue signal and enablinggeneration of a synthesis signal representative of the digital audiosignal and constituting the decoded signal, “backward” LPC analysismeans receiving said synthesis signal and enabling generation of“backward” LPC filtering parameters, discriminating means between“forward” LPC analysis and “backward” LPC analysis respectivelyreceiving, on the one hand, for discrimination control said analysischoice value and, on the other hand, the “forward” LPC filteringparameters and the “backward” LPC filtering parameters and enablingdelivery as a function of said analysis choice value, of either“forward” LPC filtering parameters, or “backward” LPC filteringparameters to said reverse filtering adaptive means as a function of thedegree of stationarity.